This invention relates to a method of treating a superconducting wire for the purpose of reducing the number of training steps performed on the coil made from the superconducting wire and an apparatus for performing such a method on wires of small and large lengths.
For generating powerful magnetic fields, it has become increasingly frequent to use coils which have superconducting properties at predetermined low temperatures and thus have no electric resistance in that condition. In such an arrangement substantially higher current intensities may be realized than in normally conducting conductors and the energy losses may be reduced to a substantial extent.
If, however, during operation involving such superconducting coils, there occurs an unintended transition from the superconducting state to the normally conducting state, then, due to the substantially higher specific resistance of the superconducting material in the normal conducting state, the resulting abruptly increasing heat losses may lead to a heat-caused destruction of the conductor. Such an occurrence is conventionally prevented by dividing the superconductors into a plurality of filaments which are embedded into a matrix made of normally conducting material such as copper. When a coil made of superconductors structured in this manner is loaded with current for the first time, a transition into the normal conducting condition may occur at a certain current load which is below the critical current intensity of the wire. Upon repeating several times this loading process, the current intensity is increased in a step-wise manner up to the critical current intensity. Thus, a phenomenon occurs which is comparable to a training effect. This phenomenon can be observed in particular in partially stabilized NbTi superconductors. The reason for this phenomenon is seen in a stress-induced microplastic deformation or shape memory efforts in NbTi, the intensity of which may be reduced by means of a pretreatment of the superconductor.
It is known to perform the above-outlined training of a superconducting coil for setting the highest possible critical current intensity by submitting the magnetic coil, to a greater or lesser number of transitions into the normally conducting state. The disadvantages of such a process reside particularly in the fact that as the size of the magnetic coil increases, the number of the necessary training steps and the period required for the individual training steps increase rapidly.
Further, a process is also known--as disclosed in U.S. Pat. No. 3,953,922--wherein a sleeve or a pin is inserted in the air core of the coil, or a coaxial ring is cylindrically arranged about the coil and by heating and/or cooling, heat-caused stresses are transmitted from such an auxiliary arrangement to the coil. It is a significant disadvantage of this arrangement that additional devices and auxiliary means are necessary which, particularly in case of coils of large dimensions, means substantial added expense. Further, during this stretching process, changes in the coil geometry may occur which can lead to a significant deterioration in the quality, because in air-core coils the field geometry in the superconducting range is determined exclusively by the coil geometry.